1. Field of the Invention
The present invention relates to a main lens member for an electron gun provided in a cathode ray tube used for example for a color tube, projector tube, or index tube and to an electron gun using the same.
2. Description of the Related Art
An uni-potential focus type electron gun provided in a cathode ray tube is for example designed with a first grid to fifth grid arranged concentrically with respect to the tube axis as the accelerating and converging electrodes with respect to a cathode for emitting electrons. The electron beam emitted from the cathode is converged on the phosphor screen due to the action of a pre-focus lens system formed by the second and third grids and a main lens system formed by the third grid, fourth grid, and fifth grid. An anode voltage is applied to the third grid and fifth grid, while a voltage close to zero is applied to the fourth grid. These cathode and first to fifth grids are fixed in place by beading of beading glass to be assembled into a single unit. The first to fifth grids are for example made of stainless steel.
Such an electron gun as the related art is susceptible to deviation in the concentricity among the third to fifth grids forming the main lens system. Accordingly, it suffers from sometimes the disadvantage of an off-axis angle of the electron beam and consequent defocusing. Also, due to the step-like nature of the potential difference among the grids, it also suffers from the disadvantages that electric discharge easily occurs among the third to fifth grids and, further, the spherical aberration of the main lens system becomes greater and the beam spot becomes larger in diameter.
The assignee of this application filed a previous U.S. patent application for an electron gun solving the above disadvantages on Dec. 27, 1993 Ser. No. 08/172,733). The electron gun of this prior application, as explained with reference to the embodiment illustrated in FIG. 2 attached to the specification of that application, has a main lens consisting of a hollow high resistance tube 3 and ring-shaped electrode films 8, 9, and 10 formed on the inner surfaces of the two ends and the inner surface of the center portion of the high resistance tube 3. The electron beam passes inside the hollow high resistance tube 3. The ring-shaped electrode films 8, 9, and are made of a ruthenium oxide-glass paste and respectively performed the functions of the third grid G.sub.3, fourth grid G.sub.4, and fifth grid G.sub.5. On the other hand, on the inner surface of the high resistance tube 3 between the electrode films 8, 9, and 10 there are formed a plurality of conductive rings 11 made of the same material as the electrode films 8, 9, and 10. The electrode films 8, 9, and 10 and the plurality of conductive rings 11 are formed in a direction perpendicular to the tube axis (Z-axis) of the high resistance tube 3.
Since the main lens of this earlier application is comprised of the hollow high resistance tube 3 and electrode films 8, 9, and 10, it is possible to design a large distance among the electrode films 8, 9, and 10, possible to made the potential gradient between the electrodes lower, and possible to reduce the spherical aberration and thereby reduce the size of the electron beam spot. Further, by providing the conductive rings 11, it is possible to make the surface potential at the inside of the high resistance tube 3 uniform (that is, possible to achieve rotational symmetry about the tube axis) and thereby increase the stability with respect to local changes in potential in the main lens system caused by stray emission ("charge up") etc.
Further, as explained with reference to another embodiment illustrated in FIG. 21 attached to the specification of the earlier application, the main lens could also have been comprised of two high resistance ceramic members 46 and a metal member 50 sandwiched between these ceramic members 46. The high resistance ceramic members 46 have the third grid G.sub.3 and a fifth grid G.sub.5 formed on them, while the metal member 50 corresponded to the fourth grid G.sub.4.
In addition, as explained with reference to still another embodiment illustrated in FIGS. 22A and 22B attached to the specification of the earlier application, the main lens could have been comprised of superposed ring-shaped members 54 made of a high resistance ceramic between which were inserted disk-shaped metal plates 55.
In the fabrication of the main lens of the embodiment illustrated in FIG. 2 of the above earlier application, however, it is necessary to form the electrode films 8, 9, and 10 and the plurality of conductive rings 11 on the inner surface of the high resistance tube. The electrode films 8, 9, and 10 and the plurality of conductive rings 11 are formed by using a rubber roller to transfer the conductive paste to the inner surface of the high resistance tube, then trimming the conductive paste or combining a negative resist material and the metal thin film vapor deposition method or the metal mask vapor deposition method, heat transfer method, dispenser method, etc.
No matter what method was used, however, it is difficult to form the electrode films 8, 9, and 10 and the plurality of conductive rings 11 on the inside surface of the high resistance tube with a high precision. In particular, when making the inside diameter of the high resistance tube 3 smaller or in the case of an electron gun for a picture tube having a plurality of holes through which electron beams pass, it is even more difficult to form the electrode films 8, 9, and 10 and the plurality of conductive rings 11 with a high precision.
In the fabrication of the main lens of the embodiments illustrated in FIG. 21 and in FIGS. 22A and 22B of the above earlier application, it is difficult to join the ceramic members 46 and metal members 50 and to join the ring-shaped members 54 and metal plates 55 since they are completely different materials.
Also, in the above earlier application, there is the problem that it is difficult to join and hold the high resistance tube 3 or joined ceramic members 46 and metal members 50 or ring-shaped members 54 and metal plates 54 constituting the main lens to other components of the electron gun.